1. Field of the Invention
The present invention relates to a laminar polymer extrusion presenting vapor, gas and/or organic liquids barrier capabilities. The invention also extends to a process and die unit capable of producing this laminar extrusion.
The laminar extrusion in accordance with the present invention is particularly useful to manufacture containers for foods, beverages, pharmaceuticals, etc.
2. Brief Description of the Prior Art
In the industry, the need for enhancement of product properties and processing behavior, and for meeting government regulations has motivated the development of materials with improved performance. Polymers and their derivatives have provided solutions to material related problems in many situations, with both economic and technical advantages. For example, polymer structures including thin layers of different polymers, a mixture of many polymers, or additives have been developed.
In particular, the use of polymers in the packaging industry is presently increasing steadily. Polymers offer many advantages over other conventionally used packaging materials. Amongst others, their low cost, relative ease of processing, flexibility, good physical properties, and light weight can be mentioned. An important obstacle in the use of polymers in this industry, however, is the high permeability of a layer made of only one polymer to various gases and solvents.
To overcome that problem, high-gas-barrier coextruded bottles have been developed and this food packaging product line is growing very rapidly. A key polymer used in the production of such bottles is ethylene vinyl-alcohol copolymer (EVOH) which is generally approved for food-contact applications. EVOH is resistant to permeation of gases such as oxygen and carbon dioxide, of vapors and of organic liquids including solvents. However it presents the shortcomings of being moisture sensitive, and having reduced oxygen permeation resistance with increasing relative humidity. Thus, during manufacture, a layer of EVOH is sandwiched between layers of one or more common polymers such as polyethylene (PE) and polypropylene (PP). PE and PP have excellent water barrier properties but low oxygen permeation resistance. The combination of EVOH and PE or PP in a multi-layer structure with the incorporation of adhesive tie-layers between EVOH and the polyolefin(s) results in structures exhibiting mechanical strength, light weight and excellent barrier properties.
The multi-layer manufacture techniques presently available require multi-layer extrusion dies, additional extruders, and appropriate adhesive tie-layers. Although multi-layer extrusion products satisfy many of the requirements in the packaging industry, they still involve important capital investments to purchase the equipment, require complex process optimization and control, and present limitations in recycling of regrind or waste products.
Polymer blends constitute an interesting alternative to multi-layer extrusion to achieve desirable product properties. They have been used extensively by the plastics industry to meet some stringent requirements of performance and cost.
Polymer blends usually include two or more polymers, mixed physically together. Compatible (soluble) blends yield polymer alloys, whereas most of the commercial blends comprise incompatible (insoluble) polymers to form a dispersion of one polymer in the other one. The behavior of a polymer blend product will depend, to a large extent, on the microstructure of the blend, reflecting the size and configuration of the dispersed phase, and the nature of the interface between the two phases.
It should be pointed out here that in the present specification and in the appended claims the term "incompatible" qualifies phases which cannot dissolve completely in each other although a certain degree of compatibility can exist between the two phases to achieve adhesion at the interface of the two phases.
As an example, U.S. Pat. No. 4,410,482 (SUBRAMANIAN) issued on Oct. 18, 1983, proposes a method for producing a laminar structure from a polymer blend.
In a first step is prepared an heterogeneous blend of 60% to 95% by weight of polyolefin, 5% to 40% by weight of a condensation polymer incompatible with the polyolefin, and 0.25% to 12% by weight of a so called compatibilizer advantageously formed by an alkylcarboxyl-substituted polyolefin.
The heterogeneous blend is heated above the highest melting point of the blend constituents. At this temperature, the blend melts so that it can be subsequently formed by stretching a body of melt blend and cooling the so stretched body until it reaches a temperature below the lowest melting point of the blend constituents. An elongation of 100 to 500 percent of the body in at least one direction is required. The body of molten blend is elongated by means of conventional equipments such as low shear and low mixing extruders, die lips, pressing rollers or platens, etc.
The polyolefin constitutes a continuous matrix phase while the condensation polymer incompatible with the polyolefin presents the form of a discontinuous distribution of thin, substantially two-dimensional, parallel and overlapping layers. The compatibilizer, namely the alkylcarboxyl-substituted polyolefin, is present between the matrix phase and the layers of condensation polymer to adhere these matrix and layers together.
In the multi-layer extrusion processes, the necessary equipment is complex and expensive, and at least one adhesive tie-layer is necessary. Another important drawback of multi-layer extrusion processes is that they do not enable recycling of the regrind and of the extrusion itself after use. Production of laminar structures by extruding a molten polymer blend simplifies the necessary equipment, but the prior art techniques require elaborate screw design and delicate control of temperature along the barrel and screw of the extruder to produce and maintain the layered structure in the screw region. Another important drawback of prior art techniques is the limited flexibility with regard to the choice of blend components and the control of distribution and morphology of the dispersed phase in the matrix.